i have a PTO Model BOM item, that has an option class, and underneath that a subcomponent. the subcomponent is also sold individually at price X. when it is sold as a part of the PTO Model item, the subcomponent is priced 10% lower (say Y).

i have defined two different price lines in the price list for the subcomponent, one line with price X and the other with Y (where Y is 90% of X). in the line with price Y, i have defined a pricing attribute > Model ID with the item number of the top level PTO Model item. the PTO Model item itself (along with the option class) is defined with a price of 0 (zero).